linux-stable.git/kernel, branch v4.19.98

tick/sched: Annotate lockless access to last_jiffies_update

2020-01-23T07:21:37+00:00

commit de95a991bb72e009f47e0c4bbc90fc5f594588d5 upstream.

syzbot (KCSAN) reported a data-race in tick_do_update_jiffies64():

BUG: KCSAN: data-race in tick_do_update_jiffies64 / tick_do_update_jiffies64

write to 0xffffffff8603d008 of 8 bytes by interrupt on cpu 1:
 tick_do_update_jiffies64+0x100/0x250 kernel/time/tick-sched.c:73
 tick_sched_do_timer+0xd4/0xe0 kernel/time/tick-sched.c:138
 tick_sched_timer+0x43/0xe0 kernel/time/tick-sched.c:1292
 __run_hrtimer kernel/time/hrtimer.c:1514 [inline]
 __hrtimer_run_queues+0x274/0x5f0 kernel/time/hrtimer.c:1576
 hrtimer_interrupt+0x22a/0x480 kernel/time/hrtimer.c:1638
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1110 [inline]
 smp_apic_timer_interrupt+0xdc/0x280 arch/x86/kernel/apic/apic.c:1135
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
 arch_local_irq_restore arch/x86/include/asm/paravirt.h:756 [inline]
 kcsan_setup_watchpoint+0x1d4/0x460 kernel/kcsan/core.c:436
 check_access kernel/kcsan/core.c:466 [inline]
 __tsan_read1 kernel/kcsan/core.c:593 [inline]
 __tsan_read1+0xc2/0x100 kernel/kcsan/core.c:593
 kallsyms_expand_symbol.constprop.0+0x70/0x160 kernel/kallsyms.c:79
 kallsyms_lookup_name+0x7f/0x120 kernel/kallsyms.c:170
 insert_report_filterlist kernel/kcsan/debugfs.c:155 [inline]
 debugfs_write+0x14b/0x2d0 kernel/kcsan/debugfs.c:256
 full_proxy_write+0xbd/0x100 fs/debugfs/file.c:225
 __vfs_write+0x67/0xc0 fs/read_write.c:494
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff8603d008 of 8 bytes by task 0 on cpu 0:
 tick_do_update_jiffies64+0x2b/0x250 kernel/time/tick-sched.c:62
 tick_nohz_update_jiffies kernel/time/tick-sched.c:505 [inline]
 tick_nohz_irq_enter kernel/time/tick-sched.c:1257 [inline]
 tick_irq_enter+0x139/0x1c0 kernel/time/tick-sched.c:1274
 irq_enter+0x4f/0x60 kernel/softirq.c:354
 entering_irq arch/x86/include/asm/apic.h:517 [inline]
 entering_ack_irq arch/x86/include/asm/apic.h:523 [inline]
 smp_apic_timer_interrupt+0x55/0x280 arch/x86/kernel/apic/apic.c:1133
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
 native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:60
 arch_cpu_idle+0xa/0x10 arch/x86/kernel/process.c:571
 default_idle_call+0x1e/0x40 kernel/sched/idle.c:94
 cpuidle_idle_call kernel/sched/idle.c:154 [inline]
 do_idle+0x1af/0x280 kernel/sched/idle.c:263
 cpu_startup_entry+0x1b/0x20 kernel/sched/idle.c:355
 rest_init+0xec/0xf6 init/main.c:452
 arch_call_rest_init+0x17/0x37
 start_kernel+0x838/0x85e init/main.c:786
 x86_64_start_reservations+0x29/0x2b arch/x86/kernel/head64.c:490
 x86_64_start_kernel+0x72/0x76 arch/x86/kernel/head64.c:471
 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:241

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Use READ_ONCE() and WRITE_ONCE() to annotate this expected race.

Reported-by: syzbot 
Signed-off-by: Eric Dumazet 
Signed-off-by: Thomas Gleixner 
Link: https://lore.kernel.org/r/20191205045619.204946-1-edumazet@google.com
Signed-off-by: Greg Kroah-Hartman

bpf: Fix incorrect verifier simulation of ARSH under ALU32

2020-01-23T07:21:32+00:00

commit 0af2ffc93a4b50948f9dad2786b7f1bd253bf0b9 upstream.

Anatoly has been fuzzing with kBdysch harness and reported a hang in one
of the outcomes:

  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  0: (85) call bpf_get_socket_cookie#46
  1: R0_w=invP(id=0) R10=fp0
  1: (57) r0 &= 808464432
  2: R0_w=invP(id=0,umax_value=808464432,var_off=(0x0; 0x30303030)) R10=fp0
  2: (14) w0 -= 810299440
  3: R0_w=invP(id=0,umax_value=4294967295,var_off=(0xcf800000; 0x3077fff0)) R10=fp0
  3: (c4) w0 s>>= 1
  4: R0_w=invP(id=0,umin_value=1740636160,umax_value=2147221496,var_off=(0x67c00000; 0x183bfff8)) R10=fp0
  4: (76) if w0 s>= 0x30303030 goto pc+216
  221: R0_w=invP(id=0,umin_value=1740636160,umax_value=2147221496,var_off=(0x67c00000; 0x183bfff8)) R10=fp0
  221: (95) exit
  processed 6 insns (limit 1000000) [...]

Taking a closer look, the program was xlated as follows:

  # ./bpftool p d x i 12
  0: (85) call bpf_get_socket_cookie#7800896
  1: (bf) r6 = r0
  2: (57) r6 &= 808464432
  3: (14) w6 -= 810299440
  4: (c4) w6 s>>= 1
  5: (76) if w6 s>= 0x30303030 goto pc+216
  6: (05) goto pc-1
  7: (05) goto pc-1
  8: (05) goto pc-1
  [...]
  220: (05) goto pc-1
  221: (05) goto pc-1
  222: (95) exit

Meaning, the visible effect is very similar to f54c7898ed1c ("bpf: Fix
precision tracking for unbounded scalars"), that is, the fall-through
branch in the instruction 5 is considered to be never taken given the
conclusion from the min/max bounds tracking in w6, and therefore the
dead-code sanitation rewrites it as goto pc-1. However, real-life input
disagrees with verification analysis since a soft-lockup was observed.

The bug sits in the analysis of the ARSH. The definition is that we shift
the target register value right by K bits through shifting in copies of
its sign bit. In adjust_scalar_min_max_vals(), we do first coerce the
register into 32 bit mode, same happens after simulating the operation.
However, for the case of simulating the actual ARSH, we don't take the
mode into account and act as if it's always 64 bit, but location of sign
bit is different:

  dst_reg->smin_value >>= umin_val;
  dst_reg->smax_value >>= umin_val;
  dst_reg->var_off = tnum_arshift(dst_reg->var_off, umin_val);

Consider an unknown R0 where bpf_get_socket_cookie() (or others) would
for example return 0xffff. With the above ARSH simulation, we'd see the
following results:

  [...]
  1: R1=ctx(id=0,off=0,imm=0) R2_w=invP65535 R10=fp0
  1: (85) call bpf_get_socket_cookie#46
  2: R0_w=invP(id=0) R10=fp0
  2: (57) r0 &= 808464432
    -> R0_runtime = 0x3030
  3: R0_w=invP(id=0,umax_value=808464432,var_off=(0x0; 0x30303030)) R10=fp0
  3: (14) w0 -= 810299440
    -> R0_runtime = 0xcfb40000
  4: R0_w=invP(id=0,umax_value=4294967295,var_off=(0xcf800000; 0x3077fff0)) R10=fp0
                              (0xffffffff)
  4: (c4) w0 s>>= 1
    -> R0_runtime = 0xe7da0000
  5: R0_w=invP(id=0,umin_value=1740636160,umax_value=2147221496,var_off=(0x67c00000; 0x183bfff8)) R10=fp0
                              (0x67c00000)           (0x7ffbfff8)
  [...]

In insn 3, we have a runtime value of 0xcfb40000, which is '1100 1111 1011
0100 0000 0000 0000 0000', the result after the shift has 0xe7da0000 that
is '1110 0111 1101 1010 0000 0000 0000 0000', where the sign bit is correctly
retained in 32 bit mode. In insn4, the umax was 0xffffffff, and changed into
0x7ffbfff8 after the shift, that is, '0111 1111 1111 1011 1111 1111 1111 1000'
and means here that the simulation didn't retain the sign bit. With above
logic, the updates happen on the 64 bit min/max bounds and given we coerced
the register, the sign bits of the bounds are cleared as well, meaning, we
need to force the simulation into s32 space for 32 bit alu mode.

Verification after the fix below. We're first analyzing the fall-through branch
on 32 bit signed >= test eventually leading to rejection of the program in this
specific case:

  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  0: (b7) r2 = 808464432
  1: R1=ctx(id=0,off=0,imm=0) R2_w=invP808464432 R10=fp0
  1: (85) call bpf_get_socket_cookie#46
  2: R0_w=invP(id=0) R10=fp0
  2: (bf) r6 = r0
  3: R0_w=invP(id=0) R6_w=invP(id=0) R10=fp0
  3: (57) r6 &= 808464432
  4: R0_w=invP(id=0) R6_w=invP(id=0,umax_value=808464432,var_off=(0x0; 0x30303030)) R10=fp0
  4: (14) w6 -= 810299440
  5: R0_w=invP(id=0) R6_w=invP(id=0,umax_value=4294967295,var_off=(0xcf800000; 0x3077fff0)) R10=fp0
  5: (c4) w6 s>>= 1
  6: R0_w=invP(id=0) R6_w=invP(id=0,umin_value=3888119808,umax_value=4294705144,var_off=(0xe7c00000; 0x183bfff8)) R10=fp0
                                              (0x67c00000)          (0xfffbfff8)
  6: (76) if w6 s>= 0x30303030 goto pc+216
  7: R0_w=invP(id=0) R6_w=invP(id=0,umin_value=3888119808,umax_value=4294705144,var_off=(0xe7c00000; 0x183bfff8)) R10=fp0
  7: (30) r0 = *(u8 *)skb[808464432]
  BPF_LD_[ABS|IND] uses reserved fields
  processed 8 insns (limit 1000000) [...]

Fixes: 9cbe1f5a32dc ("bpf/verifier: improve register value range tracking with ARSH")
Reported-by: Anatoly Trosinenko 
Signed-off-by: Daniel Borkmann 
Acked-by: Yonghong Song 
Signed-off-by: Alexei Starovoitov 
Link: https://lore.kernel.org/bpf/20200115204733.16648-1-daniel@iogearbox.net
Signed-off-by: Greg Kroah-Hartman

ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()

2020-01-23T07:21:29+00:00

commit 6b3ad6649a4c75504edeba242d3fd36b3096a57f upstream.

Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
introduced the ability to opt out of audit messages for accesses to various
proc files since they are not violations of policy.  While doing so it
somehow switched the check from ns_capable() to
has_ns_capability{_noaudit}(). That means it switched from checking the
subjective credentials of the task to using the objective credentials. This
is wrong since. ptrace_has_cap() is currently only used in
ptrace_may_access() And is used to check whether the calling task (subject)
has the CAP_SYS_PTRACE capability in the provided user namespace to operate
on the target task (object). According to the cred.h comments this would
mean the subjective credentials of the calling task need to be used.
This switches ptrace_has_cap() to use security_capable(). Because we only
call ptrace_has_cap() in ptrace_may_access() and in there we already have a
stable reference to the calling task's creds under rcu_read_lock() there's
no need to go through another series of dereferences and rcu locking done
in ns_capable{_noaudit}().

As one example where this might be particularly problematic, Jann pointed
out that in combination with the upcoming IORING_OP_OPENAT feature, this
bug might allow unprivileged users to bypass the capability checks while
asynchronously opening files like /proc/*/mem, because the capability
checks for this would be performed against kernel credentials.

To illustrate on the former point about this being exploitable: When
io_uring creates a new context it records the subjective credentials of the
caller. Later on, when it starts to do work it creates a kernel thread and
registers a callback. The callback runs with kernel creds for
ktask->real_cred and ktask->cred. To prevent this from becoming a
full-blown 0-day io_uring will call override_cred() and override
ktask->cred with the subjective credentials of the creator of the io_uring
instance. With ptrace_has_cap() currently looking at ktask->real_cred this
override will be ineffective and the caller will be able to open arbitray
proc files as mentioned above.
Luckily, this is currently not exploitable but will turn into a 0-day once
IORING_OP_OPENAT{2} land in v5.6. Fix it now!

Cc: Oleg Nesterov 
Cc: Eric Paris 
Cc: stable@vger.kernel.org
Reviewed-by: Kees Cook 
Reviewed-by: Serge Hallyn 
Reviewed-by: Jann Horn 
Fixes: 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
Signed-off-by: Christian Brauner 
Signed-off-by: Greg Kroah-Hartman

LSM: generalize flag passing to security_capable

2020-01-23T07:21:29+00:00

[ Upstream commit c1a85a00ea66cb6f0bd0f14e47c28c2b0999799f ]

This patch provides a general mechanism for passing flags to the
security_capable LSM hook. It replaces the specific 'audit' flag that is
used to tell security_capable whether it should log an audit message for
the given capability check. The reason for generalizing this flag
passing is so we can add an additional flag that signifies whether
security_capable is being called by a setid syscall (which is needed by
the proposed SafeSetID LSM).

Signed-off-by: Micah Morton 
Reviewed-by: Kees Cook 
Signed-off-by: James Morris 
Signed-off-by: Sasha Levin

tracing: Have stack tracer compile when MCOUNT_INSN_SIZE is not defined

2020-01-14T19:06:59+00:00

commit b8299d362d0837ae39e87e9019ebe6b736e0f035 upstream.

On some archs with some configurations, MCOUNT_INSN_SIZE is not defined, and
this makes the stack tracer fail to compile. Just define it to zero in this
case.

Link: https://lore.kernel.org/r/202001020219.zvE3vsty%lkp@intel.com

Cc: stable@vger.kernel.org
Fixes: 4df297129f622 ("tracing: Remove most or all of stack tracer stack size from stack_max_size")
Reported-by: kbuild test robot 
Signed-off-by: Steven Rostedt (VMware) 
Signed-off-by: Greg Kroah-Hartman

kernel/trace: Fix do not unregister tracepoints when register sched_migrate_task fail

2020-01-14T19:06:59+00:00

commit 50f9ad607ea891a9308e67b81f774c71736d1098 upstream.

In the function, if register_trace_sched_migrate_task() returns error,
sched_switch/sched_wakeup_new/sched_wakeup won't unregister. That is
why fail_deprobe_sched_switch was added.

Link: http://lkml.kernel.org/r/20191231133530.2794-1-pilgrimtao@gmail.com

Cc: stable@vger.kernel.org
Fixes: 478142c39c8c2 ("tracing: do not grab lock in wakeup latency function tracing")
Signed-off-by: Kaitao Cheng 
Signed-off-by: Steven Rostedt (VMware) 
Signed-off-by: Greg Kroah-Hartman

locking/spinlock/debug: Fix various data races

2020-01-12T11:17:05+00:00

[ Upstream commit 1a365e822372ba24c9da0822bc583894f6f3d821 ]

This fixes various data races in spinlock_debug. By testing with KCSAN,
it is observable that the console gets spammed with data races reports,
suggesting these are extremely frequent.

Example data race report:

  read to 0xffff8ab24f403c48 of 4 bytes by task 221 on cpu 2:
   debug_spin_lock_before kernel/locking/spinlock_debug.c:85 [inline]
   do_raw_spin_lock+0x9b/0x210 kernel/locking/spinlock_debug.c:112
   __raw_spin_lock include/linux/spinlock_api_smp.h:143 [inline]
   _raw_spin_lock+0x39/0x40 kernel/locking/spinlock.c:151
   spin_lock include/linux/spinlock.h:338 [inline]
   get_partial_node.isra.0.part.0+0x32/0x2f0 mm/slub.c:1873
   get_partial_node mm/slub.c:1870 [inline]
  

  write to 0xffff8ab24f403c48 of 4 bytes by task 167 on cpu 3:
   debug_spin_unlock kernel/locking/spinlock_debug.c:103 [inline]
   do_raw_spin_unlock+0xc9/0x1a0 kernel/locking/spinlock_debug.c:138
   __raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:159 [inline]
   _raw_spin_unlock_irqrestore+0x2d/0x50 kernel/locking/spinlock.c:191
   spin_unlock_irqrestore include/linux/spinlock.h:393 [inline]
   free_debug_processing+0x1b3/0x210 mm/slub.c:1214
   __slab_free+0x292/0x400 mm/slub.c:2864
  

As a side-effect, with KCSAN, this eventually locks up the console, most
likely due to deadlock, e.g. .. -> printk lock -> spinlock_debug ->
KCSAN detects data race -> kcsan_print_report() -> printk lock ->
deadlock.

This fix will 1) avoid the data races, and 2) allow using lock debugging
together with KCSAN.

Reported-by: Qian Cai 
Signed-off-by: Marco Elver 
Cc: Andrew Morton 
Cc: Linus Torvalds 
Cc: Paul E. McKenney 
Cc: Peter Zijlstra 
Cc: Thomas Gleixner 
Cc: Will Deacon 
Link: https://lkml.kernel.org/r/20191120155715.28089-1-elver@google.com
Signed-off-by: Ingo Molnar 
Signed-off-by: Sasha Levin

bpf: Fix passing modified ctx to ld/abs/ind instruction

2020-01-12T11:17:04+00:00

commit 6d4f151acf9a4f6fab09b615f246c717ddedcf0c upstream.

Anatoly has been fuzzing with kBdysch harness and reported a KASAN
slab oob in one of the outcomes:

  [...]
  [   77.359642] BUG: KASAN: slab-out-of-bounds in bpf_skb_load_helper_8_no_cache+0x71/0x130
  [   77.360463] Read of size 4 at addr ffff8880679bac68 by task bpf/406
  [   77.361119]
  [   77.361289] CPU: 2 PID: 406 Comm: bpf Not tainted 5.5.0-rc2-xfstests-00157-g2187f215eba #1
  [   77.362134] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
  [   77.362984] Call Trace:
  [   77.363249]  dump_stack+0x97/0xe0
  [   77.363603]  print_address_description.constprop.0+0x1d/0x220
  [   77.364251]  ? bpf_skb_load_helper_8_no_cache+0x71/0x130
  [   77.365030]  ? bpf_skb_load_helper_8_no_cache+0x71/0x130
  [   77.365860]  __kasan_report.cold+0x37/0x7b
  [   77.366365]  ? bpf_skb_load_helper_8_no_cache+0x71/0x130
  [   77.366940]  kasan_report+0xe/0x20
  [   77.367295]  bpf_skb_load_helper_8_no_cache+0x71/0x130
  [   77.367821]  ? bpf_skb_load_helper_8+0xf0/0xf0
  [   77.368278]  ? mark_lock+0xa3/0x9b0
  [   77.368641]  ? kvm_sched_clock_read+0x14/0x30
  [   77.369096]  ? sched_clock+0x5/0x10
  [   77.369460]  ? sched_clock_cpu+0x18/0x110
  [   77.369876]  ? bpf_skb_load_helper_8+0xf0/0xf0
  [   77.370330]  ___bpf_prog_run+0x16c0/0x28f0
  [   77.370755]  __bpf_prog_run32+0x83/0xc0
  [   77.371153]  ? __bpf_prog_run64+0xc0/0xc0
  [   77.371568]  ? match_held_lock+0x1b/0x230
  [   77.371984]  ? rcu_read_lock_held+0xa1/0xb0
  [   77.372416]  ? rcu_is_watching+0x34/0x50
  [   77.372826]  sk_filter_trim_cap+0x17c/0x4d0
  [   77.373259]  ? sock_kzfree_s+0x40/0x40
  [   77.373648]  ? __get_filter+0x150/0x150
  [   77.374059]  ? skb_copy_datagram_from_iter+0x80/0x280
  [   77.374581]  ? do_raw_spin_unlock+0xa5/0x140
  [   77.375025]  unix_dgram_sendmsg+0x33a/0xa70
  [   77.375459]  ? do_raw_spin_lock+0x1d0/0x1d0
  [   77.375893]  ? unix_peer_get+0xa0/0xa0
  [   77.376287]  ? __fget_light+0xa4/0xf0
  [   77.376670]  __sys_sendto+0x265/0x280
  [   77.377056]  ? __ia32_sys_getpeername+0x50/0x50
  [   77.377523]  ? lock_downgrade+0x350/0x350
  [   77.377940]  ? __sys_setsockopt+0x2a6/0x2c0
  [   77.378374]  ? sock_read_iter+0x240/0x240
  [   77.378789]  ? __sys_socketpair+0x22a/0x300
  [   77.379221]  ? __ia32_sys_socket+0x50/0x50
  [   77.379649]  ? mark_held_locks+0x1d/0x90
  [   77.380059]  ? trace_hardirqs_on_thunk+0x1a/0x1c
  [   77.380536]  __x64_sys_sendto+0x74/0x90
  [   77.380938]  do_syscall_64+0x68/0x2a0
  [   77.381324]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
  [   77.381878] RIP: 0033:0x44c070
  [...]

After further debugging, turns out while in case of other helper functions
we disallow passing modified ctx, the special case of ld/abs/ind instruction
which has similar semantics (except r6 being the ctx argument) is missing
such check. Modified ctx is impossible here as bpf_skb_load_helper_8_no_cache()
and others are expecting skb fields in original position, hence, add
check_ctx_reg() to reject any modified ctx. Issue was first introduced back
in f1174f77b50c ("bpf/verifier: rework value tracking").

Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
Reported-by: Anatoly Trosinenko 
Signed-off-by: Daniel Borkmann 
Signed-off-by: Alexei Starovoitov 
Link: https://lore.kernel.org/bpf/20200106215157.3553-1-daniel@iogearbox.net
Signed-off-by: Greg Kroah-Hartman

ftrace: Avoid potential division by zero in function profiler

2020-01-09T09:19:03+00:00

commit e31f7939c1c27faa5d0e3f14519eaf7c89e8a69d upstream.

The ftrace_profile->counter is unsigned long and
do_div truncates it to 32 bits, which means it can test
non-zero and be truncated to zero for division.
Fix this issue by using div64_ul() instead.

Link: http://lkml.kernel.org/r/20200103030248.14516-1-wenyang@linux.alibaba.com

Cc: stable@vger.kernel.org
Fixes: e330b3bcd8319 ("tracing: Show sample std dev in function profiling")
Fixes: 34886c8bc590f ("tracing: add average time in function to function profiler")
Signed-off-by: Wen Yang 
Signed-off-by: Steven Rostedt (VMware) 
Signed-off-by: Greg Kroah-Hartman

exit: panic before exit_mm() on global init exit

2020-01-09T09:19:02+00:00

commit 43cf75d96409a20ef06b756877a2e72b10a026fc upstream.

Currently, when global init and all threads in its thread-group have exited
we panic via:
do_exit()
-> exit_notify()
   -> forget_original_parent()
      -> find_child_reaper()
This makes it hard to extract a useable coredump for global init from a
kernel crashdump because by the time we panic exit_mm() will have already
released global init's mm.
This patch moves the panic futher up before exit_mm() is called. As was the
case previously, we only panic when global init and all its threads in the
thread-group have exited.

Signed-off-by: chenqiwu 
Acked-by: Christian Brauner 
Acked-by: Oleg Nesterov 
[christian.brauner@ubuntu.com: fix typo, rewrite commit message]
Link: https://lore.kernel.org/r/1576736993-10121-1-git-send-email-qiwuchen55@gmail.com
Signed-off-by: Christian Brauner 
Signed-off-by: Greg Kroah-Hartman